Preheat oven to 400 degrees F.
Heat oil in large skillet over medium heat. Add sausage and cook for 3 minutes, or until browned and cooked through, breaking up the meat as it cooks with a wooden spoon. Add onion and mushrooms and cook 2 minutes, until soft. Add tomato sauce, olives and bring to a simmer, about 2 minutes. Remove from heat and transfer sauce to the bottom of a 13 by 9-inch baking dish. Top with mozzarella cheese.
In a medium bowl, whisk together eggs, milk and oil until blended. Whisk in flour and salt until well blended. Pour mixture over sauce in pan. Sprinkle with Parmesan. Bake 25 minutes, or until golden brown. Let stand for 5 minutes before slicing and serving.